Optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes, are solid state devices that generate light in response to excitation signals. Traditionally, the most efficient LEDs emit light having a peak wavelength in the red region of the light spectrum. However, a type of LED based on Gallium nitride (GaN) has been developed that can efficiently emit light having a peak wavelength in the blue region of the light spectrum. This LED may provide significantly greater light output than traditional LEDs. Moreover, since blue light has a shorter wavelength than red light, the blue light generated by the GaN-based LED can be readily converted to produce light having a longer wavelength. This efficient conversion increases the likelihood that marketable "white light" LEDs can be fabricated. GaN-based LEDs are also fabricated to generate green light.
In an exemplary known GaN-based LED, a light-emitting semiconductor structure is formed on a sapphire substrate. The semiconductor structure includes an n-type GaN region and a p-type GaN region. These two regions are epitaxially grown. Typically, metalorganic vapor phase epitaxy is used. The p-type GaN may be formed using magnesium (Mg) or zinc (Zn) as a dopant. Other layers may also be included. For example, a buffer layer may be formed between the sapphire substrate and the semiconductor structure to serve as a transition layer that promotes adhesion between the sapphire substrate and the GaN material. The buffer layer may be formed of aluminum nitride (AIN).
In operation, light is generated in response to applying an excitation signal to the p-type GaN region and the n-type GaN region. Thus, ohmic contacts must be formed on these two regions. A concern is that an acceptable low resistance ohmic contact for the p-type GaN material is difficult to fabricate. A number of different structural arrangements and fabrication methods have been tested for forming an acceptable contact region for the p-doped GaN region. Preferably, the contact is light transmissive, so that generated light may escape through the contact. Bi-metal light transmissive contacts are described in Japanese Laid-Open Patent Application (Kokai) Nos. 10-135515 to Shibata and 10-209500 and 10-209493 to Kamimura et al. While other materials are identified in the three references, the preferred materials for forming the contacts are either cobalt (Co) and gold (Au) or nickel (Ni) and Au. A first metal (e.g., Ni or Co) is deposited on the surface of the p-type GaN material. A second metal (e.g., Au) is then deposited on the first metal. The two metals are heat treated in an ambient containing oxygen, causing the first metal to oxidize and causing the second metal to penetrate the first metal and reach the GaN material. If the first metal is Ni, the heat treatment will form a phase of NiO, but the Au remains in the metallic state.
While the known light-transmissive contacts for p-type GaN layers of optoelectronic devices achieve acceptable performance with respect to transmissivity and lateral conductivity along the surfaces of the layers, further improvements are desired. What is needed is a method of forming a light-transmissive contact that has desirable optical and electrical characteristics when applied to a p-type GaN layer.